David Cameron faced further pressure to consider military intervention to prevent genocide in Iraq as the Government stepped up the armed forces' involvement in the humanitarian relief effort.

A "small number" of RAF Chinook helicopters is being sent to the region to increase the options available as efforts continued to ease the plight of tens of thousands of Yazidis trapped in searing heat on a mountain by Islamic State (IS) forces.

The Government has already sent RAF Tornado jets equipped with sophisticated surveillance equipment to the region to help gather intelligence about the situation on Mount Sinjar and C-130 transport planes have been involved in air drops of water and other supplies for the desperate Yazidis.

The UK has also committed to transport military equipment to resupply Kurdish forces which have been outgunned by IS, in an effort to help provide protection for refugees and humanitarian operations.

But former military commanders with experience of the conflict in Iraq urged the Prime Minister to go further and join the United States in carrying out air strikes against IS targets.

Colonel Tim Collins, noted for the inspirational speech he delivered to troops on the eve of the 2003 Iraq War, warned in the Daily Telegraph that ancient civilisations in Iraq could be "extinguished" unless there was action and dismissed the aid drops as a "pebble in the ocean" compared with what was required.

He claimed the Government had "left for lunch" and politicians refused to accept the "moral obligation" to take action.

He said the British should arm and train the Kurdish Peshmerga fighters and added: " We should also be taking part in air strikes and urging our coalition partners including Turkey, France, Saudi Arabia and the United Arab Emirates to take part."

General Sir Mike Jackson, who was the professional head of the army during the Iraq War, told the newspaper: "Given our history over recent years in Iraq, we have a moral duty to do what we can on humanitarian grounds. I would have no difficulty at all in saying that we should be alongside the United States and up the British ante to the use of airpower, on humanitarian grounds."

Senior Christians also called on the Prime Minister to do more, with the chaplain to the Commons Speaker John Bercow saying military intervention may be necessary.

The Rev Rose Hudson-Wilkin said the world had to respond to the crisis in the country and "maybe we need to go to the extent of military action".

Ms Hudson-Wilkin told Channel 4 News: "W hen you hear of such tragedy unfolding before your very eyes, you cannot help but see this is genocide. And I just think that Britain, the European Union, the world community, we have got to respond."

She added: "Maybe we need to go to the extent of military action, I don't know. But we need to somehow go to the assistance of these people."

Meanwhile Cardinal Vincent Nichols, Archbishop of Westminster and the leader of the Roman Catholic church in England and Wales, said he wanted to see a greater aid effort.

Downing Street has resisted calls for British forces to join the US in launching attacks against IS, and has also rejected demands for Parliament to be recalled to debate the crisis.

Mr Cameron has remained on holiday with his family in Portugal, but is expected to be back at work in No 10 on Thursday.

A No 10 spokeswoman said: "Our focus remains the humanitarian situation, particularly those trapped on Mount Sinjar. Three UK aid drops have now taken place, with two C-130s delivering 3180 reusable water containers, filled with a total of 15,900 litres of clean water, and 816 solar lanterns overnight.

"We will continue with these deliveries. And, as part of our efforts to alleviate humanitarian suffering in Iraq, we are sending a small number of Chinook helicopters to the region for use if we decide we need further humanitarian relief options.

"Meanwhile urgent planning to get those trapped on the mountainside to safety will continue in the coming days between ourselves and US, the Kurdish authorities and other partners."

As the deployment of Chinooks was confirmed, the dangers faced by aircraft was illustrated after an Iraqi military helicopter crashed during an aid flight.

Reports suggested the Mi-17 helicopter was downed after too many people tried to climb aboard in an effort to flee Mount Sinjar.

As well as the situation on the mountain, around a quarter of a million Iraqis from religious minorities have already fled their homes in the face of "convert or die" ultimatums from the advancing militants, with women executed or taken as slaves and teenagers sexually assaulted, a United Nations report concluded.

The UN's special rapporteur on minority issues, Rita Izsak, urged: "All possible measures must be taken urgently to avoid a mass atrocity and potential genocide within days or hours - civilians need to be protected on the ground and escorted out of situations of extreme peril."

The UN refugee agency said up to 35,000 people had managed to reach Iraqi Kurdistan's Dohuk region after escaping from Mount Sinjar.

"The new arrivals are exhausted, dehydrated and many have suffered sun or heat stroke, with the daily temperatures reaching 40 to 45 degrees Celsius," UN High Commissioner for Refugees spokesman Adrian Edwards.

A further 10,000 to 15,000 had arrived in Syria, but as many as 30,000 remained on the mountain.

Labour's Diane Abbott and Tory Phillip Lee added their voices to calls for MPs to cut short their summer recess to return to Westminster.

Ms Abbott told BBC2's Newsnight: "We need an international military effort ... it needs to go to the Security Council. This was what was wrong with the original Iraq war, that it wasn't a general international effort.

"I do not believe Parliament will vote for another unilateral American-British strike."

She said "increasing numbers of MPs" wanted the Commons to be recalled.

Dr Lee told Newsnight: "I think this is the biggest challenge we have faced in 70 years. This could become a regional conflagration and much bigger."